Dude here's what you do: have her lie on her belly...then you lie on top of her (you might need to prop yourself up on an elbow depending how much of your weight she can handle...but I'm a big guy and I don't really have to support much of my body weight at all) then simply reach under her body, trap your hand between her and the mattress, and rub the clit. Done it dozens of times. Really awesome position. They'll start grinding their ass into your pelvis. Remember you guys should both be laid out completely (not doggy...you need Dikembe Motumbo arms to really work the clit doing doggy) your legs either inside or outside hers depending on penetration concerns. Let me know if it works out!

Multiple positions. The one described directly above me is really good. You can also spoon with her and do it, that's pretty easy.

Normal doggystyle is really good too for stimulating the clit.

You can stimulate it if she has her legs on your shoulders, you can also do it if she's on top, which is also an easier way to make her g-spot orgasm.

But even if you can't directly stimulate the clit in some positions girls still love it. Like when a girl has her legs above her head and you are just straight laying the pipe and beating the pussy up, and it's straight aggression, they LOVE it.

You standing up, her lying down, any position where you're not directly on top of her.

Like, the edge of the bed, a couch, a bench, the coffee table, the dining table, the kitchentop, anything where you're leaning in a bit, but not putting your weight on her.

Make sure she's on something comfortable, and if you have to raise her up, use pillows, and adjust yourself accordingly.

Also know her "speed" when it comes to rubbing, because not all of them like it fast and hard or that slow. It helps when she guides your hand, and in a way it's a good distraction that helps you go a little bit longer.

Grab a vibrating bullet. They're like $20 at the sex toy store. Use it in your forplay on her clit and get her really used to it and loving it but don't get her off w/ it just get her close. When you start to have sex in missionary position reach over and grab it again and hold it on her clit. After about 30 seconds of this (as your fucking her very slow) grab her hand and get her to hold it on her clit since she knows where the best spot is for it to sit. As long as he is comfortable w/ you and very aroused, she should just take it over. It's a great way to start to teach a girl to orgasm while actually having sex.